Electronic voltmeter and ohmmeter



Aug. 29, 1944. BANKER 2,356,733

ELECTRONIC VOLTMETER AND OHMMETER Filed April 24, 1943 W INVENTOR.

x421 WWW Patented Aug. 29, 1944 ELECTRONIC VOLTMETER AND OHMIKETER- John Richard Banker, Passaic, N. 1., assignor to Allen B. Dn Mont Laboratories, Inc., Passaic, N. J., a corporation of Delaware Application April 24, 1948, Serial No. 484,499

12 Claims.

This invention relates to an electronic circuit that may be used as a stable vacuum tube bridge.

In carrying out the invention for direct current measurements, a bridge is used that has avacuum tube in one of its arms, and in carrying it out for alternating current measurements, a bridge that has a vacuum tube in one or its arms and a rectifier are used. A source of constant voltage is needed for operating the device, or a voltage regulator is needed where the voltage from the source fluctuates.

The invention may be understood from the description in connection with the accompanying drawing, in which:

Fig. 1 is a diagram of connection for direct current use; and

Fig. 2 is a modified diagram where alternating current is to be used in making the measurements. I

In the drawing, reference character I indicates a terminal to which one side 01' direct current voltage E may be connected, the other side being connected to ground. A potentiometer resistance or voltage divider 2 which may be provided with fixed taps is connected between terminal I and ground. A lead 3 extends from the sliding contact 4 on resistance 2 to the grid 8 of the vacuum tube 6. The cathode 1 of this tube is connected by resistance 9 to the plate 01' vacuum tube III which has its cathode loaded with or biased by resistance II. The grid I2 of tube I0 is connected by lead II to the other end of the resistance II which is connected to the negative side of a direct current potential that is applied to the device. The positive side of this potential is connected to the plate ll of the tube l and a voltage regulator I5--I6 with no grounded point between them is provided to keep a constant potential between the leads I1 and I8. The resistances Is and 20 which may or may not be of equal value are connected in series between leads I1 and Il.

A sliding contact 22 is provided for the resistance I and is connected through a multiplying resistance 23 to one side of the meter M, the other side of this meter and the connection between resistances I9 and 20 being grounded as indicated at 24.

The connections described above constitute a bridge circuit, the tube 6 corresponding to the unknown arm, tube It to the standard arm, and resistances II and 20 to the ratio arms. The meter M indicates when a balanced condition exists with no signal at I and reads in accordance with increase of signal at I. The cathodes of tubes 8 and I II should be heated from the same source, thus causing the tube III to regulate for any change in the heater potential of tube 8. The bias resistance II is of such a size that the operating characteristics of the tube I. are the same or nearly the same as those of tube 8. The bridge can be easily balanced by means of sliding contact 22.

In the modification shown in Fig. 2 for measurements where alternating currents are used, similar parts are indicated by the same reference characters as before. In this modification the cathode 1 0! tube 6 is coupled by condenser 28 to the plate ll oi diode 21, the cathode of this diode being grounded and also connected by lead 28 to the plate of diode 29 so that the emission currents to the cathodes of these tubes are balanced. Resistance III is connected between plates ii of diode 21 and meter M. Resistance 22 is connected between the cathode It of diode 29 and this meter M.

The operation is as follows:

The voltage regulators It and IG- and resistances IO and 20 and ground 24 keep the potentials on leads 34 and 25 constant and at equal opposite polarities. The contact 22 is adjusted so that the meter M reads zero when no signal is applied at I. When a direct current potential or signal is applied at I in Fig. 1 the increased current through resistance 9 increases the potential at the point where contact 22 connects with resistance 9, thus causing a reading of the meter M due to the how of current through the resistance 22 because of the increased potential at the point where the contact 22 contacts with the resistance 9. The stronger the signal is that is applied at the point I the larger the reading 01 meter M is.

When an alternating current potential is applied to the terminal I in Fig. 2, a corresponding amplified potential appears on the cathodel of tube 6 due to current from lead 34 through tube 8. The cathode 1 is coupled by condenser 2 to the Plate SI of diode 21 where rectification takes place. Direct current from cathode 1 is blocked by the condenser 28. The alternating current passing through condenser 26 has its positive component removed by diode 21 leaving only unidirectional current which passes through resistance a balancing resistance in the cathode circuit or tube I. Since the condenser 20 operates as a blocking condenser which removes direct current from the plate ll of diode 21, only the alternating component of the current from tube 0 is left to be rectified and pass through the meter M to produce the reading thereon.

The meter M may be calibrated to read in volts, or it may be used as an ohmmeter, when using either direct or alternating current.

What is claimed is:

1. In a device or the character described, a bridge comprising two vacuum tubes connected in series, each tube having an anode, a grid and a cathode, a resistance between the cathode of one of said tubes and the plate or the other one, a resistance in the cathode circuit or said other tube, and a meter adiustably connected between a point on said first named resistance and ground.

2. The device of claim 1, in which the meter connection is adjustable on said first named resistance.

3. The device or claim 1, in which direct current potentials of opposite polari ies are connected to the plate of the first and the cathode of the last tube.

4. The device or claim 1, in which direct current potentials of opposite and equal polarities are connected to the plate 01' the first and the cathode of the last tube.

5. The device of claim 1, in which a resistance is in the meter connection.

6. The device of claim 1, in which a condense is in the meter connection.

7. The device of claim 1, in which the bridge has a vacuum tube in each of two arms thereof and a resistance in each one of the other arms.

8. The device oi claim 1, in which there is a bridge having a vacuum tube in each of two arms thereof and resistances of equal magnitude in the other two arms of said bridge.

9. The device of claim 1, in which a condenser is connected in series with said meter.

10. The device or claim 1, in which a condenser is connected in series with said meter and a diode is connected to said condenser.

11. The device 0! claim 1, in which a condenser and resistance are connected in series with said meter.

12. The device of claim 1, in which a condenser is connected in series with said meter and two diodes in opposition are connected to said condenser.

JOHN RICHARD BANKER. 

